Using PIVOT and UNPIVOT

You can use the PIVOT and UNPIVOT relational operators to change a table-valued expression into another table. PIVOT rotates a table-valued expression by turning the unique values from one column in the expression into multiple columns in the output, and performs aggregations where they are required on any remaining column values that are wanted in the final output. UNPIVOT performs the opposite operation to PIVOT by rotating columns of a table-valued expression into column values.

Note

When PIVOT and UNPIVOT are used against databases that are upgraded to SQL Server 2005 or later, the compatibility level of the database must be set to 90 or higher. For information about how to set the database compatibility level, see sp_dbcmptlevel (Transact-SQL).

The syntax for PIVOT provides is simpler and more readable than the syntax that may otherwise be specified in a complex series of SELECT...CASE statements. For a complete description of the syntax for PIVOT, see FROM (Transact-SQL).

A common scenario where PIVOT can be useful is when you want to generate cross-tabulation reports to summarize data. For example, suppose you want to query the PurchaseOrderHeader table in the AdventureWorks2008R2 sample database to determine the number of purchase orders placed by certain employees. The following query provides this report, ordered by vendor.

This means that the unique values returned by the EmployeeID column themselves become fields in the final result set. Therefore, there is a column for each EmployeeID number specified in the pivot clause: in this case employees 164, 198, 223, 231, and 233. The PurchaseOrderID column serves as the value column, against which the columns returned in the final output, which are called the grouping columns, are grouped. In this case, the grouping columns are aggregated by the COUNT function. Notice that a warning message appears that indicates that any null values appearing in the PurchaseOrderID column were not considered when computing the COUNT for each employee.

Important

When aggregate functions are used with PIVOT, the presence of any null values in the value column are not considered when computing an aggregation.

UNPIVOT performs almost the reverse operation of PIVOT, by rotating columns into rows. Suppose the table produced in the previous example is stored in the database as pvt, and you want to rotate the column identifiers Emp1, Emp2, Emp3, Emp4, and Emp5 into row values that correspond to a particular vendor. This means that you must identify two additional columns. The column that will contain the column values that you are rotating (Emp1, Emp2,...) will be called Employee, and the column that will hold the values that currently reside under the columns being rotated will be called Orders. These columns correspond to the pivot_column and value_column, respectively, in the Transact-SQL definition. Here is the query.

Notice that UNPIVOT is not the exact reverse of PIVOT. PIVOT performs an aggregation and, therefore, merges possible multiple rows into a single row in the output. UNPIVOT does not reproduce the original table-valued expression result because rows have been merged. Besides, null values in the input of UNPIVOT disappear in the output, whereas there may have been original null values in the input before the PIVOT operation.

The Sales.vSalesPersonSalesByFiscalYears view in the AdventureWorks2008R2 sample database uses PIVOT to return the total sales for each salesperson, for each fiscal year. To script the view in SQL Server Management Studio, in Object Explorer, locate the view under the Views folder for the AdventureWorks2008R2 database. Right-click the view name, and then select Script View as.